Although conventional propylene polymer materials have long been used in a variety of products and processes, applications such as extrusion coating, blow molding, foam formation, and thermoforming require propylene polymer materials possessing high melt strength. Generally, melt strength in polymers is improved by increasing molecular weight, broadening molecular weight distribution (for a particular molecular weight) and increasing levels of polymer branching. Molecular weight and molecular weight distribution can be modified in the polymerization process itself by choosing particular process conditions and catalyst type. However, typical propylene polymer resins, even those having high molecular weight and broad molecular weight distribution often cannot provide commercially desired levels of melt strength without additional processing. Techniques to improve melt strength have included irradiation of conventional flake polypropylene in reduced-oxygen environments, as described, for example, in U.S. Pat. Nos. 4,916,198, 5,047,485, 5,414,027, 5,541,236, 5,554,668, 5,591,785, 5,731,362, and 5,804,304. These irradiation methods increase propylene polymer melt strength by creating polymer radicals during irradiation which then re-combine to form long-chain branches in the reduced oxygen environment. Irradiation of syndiotactic and atactic metallocene-derived polymers has been described in U.S. Pat. Nos. 5,200,439 and 6,306,970 respectively. Irradiation of material having a Mw/Mn less than 2 generated by fractionation of conventional polypropylene has been described in the Journal of Applied Polymer Science, Vol. 11, pp 705-718 (1967).
Other techniques for improving melt strength include irradiating propylene polymer material in air, such as those described in U.S. Pat. No. 5,439,949, however, the increased oxygen levels favor chain scission reactions at the expense of branching reactions, which requires irradiation doses at or above the gelation point, thereby risking product quality and homogeneity. Irradiating pellets of polymer material in air, as described in U.S. Patent Publication Number 2006/0167128, has been attempted to limit oxygen exposure, however, melt strength may still be adversely affected by chain scission occurring at the outer surface of the pellets.
Phenolic antioxidants have long been used to improve polymer stability under elevated temperature conditions, such as those typically experienced during extrusion, or during extended periods of storage. However, their use in irradiated compositions undermines enhanced melt strength by scavenging free radicals, thereby reducing the number of polymeric free radicals available to recombine to form long-chain branches. Moreover, irradiation of phenolic antioxidant-containing polymers can result in the formation of degradation products that impart undesirable color. Non-phenolic stabilizers have been used in the irradiation of conventional polyolefin materials to avoid such problems, as described in U.S. Pat. No. 6,664,317 and U.S. Provisional Patent Application No. 60/937,649.
A significant challenge associated with production of high melt strength propylene materials via irradiation is the low melt flow rates typically required in the starting material to be irradiated. Low melt flow material (high viscosity) is normally used to ensure that the viscosity after irradiation is still sufficient for the needs of the application, as well as to provide long-chain radicals to help in melt strength development. However, such low melt flow rate material is also more difficult to process in plant equipment, and can result in production loss. Therefore, a continuing need exists for processes that produce irradiated propylene polymers having an improved melt strength/melt flow relationship.
Accordingly, it has unexpectedly been found that improved melt strength, as measured by melt tension, can be obtained by irradiating extrudates of compositions containing propylene polymers having a low polydispersity index and a non-phenolic stabilizer, in reduced oxygen environments. In particular, at equivalent melt flow rates in the irradiated material, superior melt tension values can be obtained. Alternately, for a particular melt tension in the irradiated material, higher melt flow rate starting material can be used.